The Mouse King is the villain in Tchaikovsky's ballet "The Nutcracker". He is responsible for casting the spell on the Prince that changed him into the Nutcracker in the first place. The Nutcracker must kill the Mouse King in battle so the enchantment can be broken. He does so on Christmas Eve night with the help of Clara, the heroine of the ballet, after the toys come to life at midnight. (In most productions, this is all part of Clara's dream after the Christmas party, but in some versions of it, it really happens.)
Sometimes, as in the Bolshoi Ballet production, the Mouse King survives the battle and returns in the second act, when he is then killed by the Nutcracker Prince. In Baryshnikov's 1977 production, the drunken party guest who breaks the Nutcracker during the party becomes the Mouse King in Clara's dream.
In E.T.A. Hoffmann's tale "The Nutcracker and the Mouse King", on which the ballet is based, the Nutcracker does not turn back into a Prince after he kills the Mouse King. He only does so after Marie (Clara in the ballet) declares that she will always love the Nutcracker even though he is ugly.